A scanning electron microscope forms an image of a sample by detecting electrons emitted from the sample as the beam scans the sample surface. A scanning electron microscope can also alter a sample by inducing a chemical reaction on the surface as the beam scans across the surface. For example, electrons in the beam can initiate a reaction of a gaseous precursor adsorbate, which can decompose to deposit material onto the surface or can etch the sample surface by combining with the surface material to form a volatile compound, which is eventually removed by the vacuum pump. The beam can also initiate a reaction directly in the sample, such as when an electron beam is used to expose a photoresist.
The resolution of electron beam processing, whether in imaging or inducing a reaction, depends in part upon the diameter of the beam as it impacts the surface. The smaller the electron beam diameter, the smaller the region from which secondary electrons will be emitted for imaging or the smaller the region that will be chemically altered.
As electrons in the primary beam impact the sample surface, they can cause the emission of other electrons, referred to as secondary electrons. The primary electrons also penetrate below the sample surface, and are backscattered from the sample. The depth of penetration and the number of backscattered electrons depend on the energy of the electrons in the primary beam and on the sample material. Backscattered electrons that return to the surface can cause the emission of additional secondary electrons, referred to as “type II” secondary electrons. Because of the scattering of the primary beam electrons in the sample, the secondary electrons and backscattered electrons are emitted from a surface area that is larger than the area of the primary beam impact, thereby reducing the resolution of electron beam processing. Secondary electron transport inside the sample prior to emission also broadens the area from which the secondary electrons are emitted from the surface.
The beam spot size is also limited by several types of aberration, the largest of which in many applications is chromatic aberration. Electrons coming from the source do not all have the same energy, and the chromatic aberration occurs because the lenses in the focusing column focus electrons having different energies at different places. One way to reduce the chromatic aberration is to use a small aperture in the beam path that stops electrons that are not tightly focused, thereby eliminating electrons having energies that deviate by a certain amount from the mean beam energy. Unfortunately, as the aperture gets smaller, diffraction effects caused by the aperture increase and cause the beam to spread out. Thus, as one reduces the chromatic aberration, one increases the beam spread due to diffraction.
It has been considered that scanning electron microscopes were reaching their limits of usefulness as microstructures were produced in the sub-10 nanometer range. In “Does SEM Have a Future?”, Semiconductor International, Dec. 27, 2007, Senior Editor Alexander E. Braun questions whether SEMs will be useful as structures are formed having dimensions of tens of nanometer and smaller. The article suggests that the trade offs between a large convergence angle to reduce diffraction effects and a small convergence angle to reduce chromatic aberration appeared to limit the usefulness of an SEM for structures in the nanometer range. Dr. Braun suggests two alternatives to extend the usefulness of SEMs: Use of aberration correction or use of higher energies. According to Dr. Braun, aberration correctors are complex devices with forty eight or sixty four active elements, and an exceeding small depth of field. It is known that many of the settings on the numerous active elements of the correctors are specific to a particular beam condition; minor changes in the beam parameters require time-consuming adjustments of many elements. While an aberration correction can be used to obtain high resolution images in a research environment, the requirement to readjust the many elements for any change in the imaging conditions makes aberration correctors impractical for routine SEM operation.
One proposed solution is to use helium ions, instead of electrons, because the helium ions have a shorter wavelength and therefore have smaller diffraction effects. Helium ions, however, are much heavier than electrons and cause more damage to the sample. Furthermore, helium ion microscopy is an immature technology with reliability problems that inhibit applications beyond research in light ion microscopy.
One application for electron beams is to initiate a chemical reaction of gaseous precursor molecules adsorbed on a sample surface to etch a sample or to deposit a material on a sample. The use of a charged particle beam to initiate a chemical reaction with a precursor is referred to as “beam chemistry.” For many applications, it is desirable to be able to use beam chemistry to fabricate high aspect ratio structures, that is, structures that are relatively deep or tall compared to their width. There are multiple competing mechanisms inherent in the charged particle beam deposition process that tend to broaden the structures produced, thereby reducing the aspect ratio.
Electrons are emitted from the sidewalls of the structures being fabricated upon impact of the primary electron beam. These electrons decompose the precursor adsorbates and cause etching or deposition on the sides of structures, making holes and deposited structures wider. The literature shows numerous results pointing to improvements in aspect ratio with increasing beam energy.
The aspect ratio of structures fabricated by beam chemistry is also affected by precursor adsorbate depletion. Depletion occurs because precursor adsorbates are consumed in the chemical reactions that cause etching or deposition, and the adsorbates are replenished at a finite rate given by the local precursor arrival rate at the sample surface. Depletion affects nanostructure aspect ratios because the electron flux that causes depletion is not constant across the sample surface. The electron flux has a maximum at the electron beam axis, and decreases with increasing distance from the beam axis. In particular, the electron flux is typically relatively low at the sidewalls and relatively high at the top surface of a structure being grown by beam chemistry. Hence, adsorbate depletion is greatest at the top surface, as is the consequential suppression of the vertical growth rate. Preferential suppression of the vertical growth rate causes a reduction in the aspect ratio of a structure grown by beam chemistry.
Thus, the industry requires a practical system for producing fine structures using a charged particle beam.